The present disclosure is directed to a drill pipe unthreading apparatus and particularly one which finds use in unthreading drill pipe which has become stuck. Assume, for purposes of illustration, that a drill string is in a well borehole and has drilled to 10,000 feet. Assume further, for purposes of description, that the drill pipe is stuck at a depth of 9,000 feet along the drill string. Sticking can arise as a result of many difficulties such as collapse of the hole, perhaps drift in the direction of the well which leads to key seating, or other conditions. When it becomes stuck, one procedure in retrieval is to rotate in the counterclockwise direction with the hope that the drill string will unthread above the stuck location. Sometimes, a bumper sub is threaded to the retrieved portion of the drill string with a hope that it can be threaded to the remaining string of pipe in the well to enhance jarring action. Obviously, the bumper sub should be as close as possible to the stuck point. If the drill string is stuck at 9,000 feet, and each drill pipe is precisely thirty feet in length, this means that 300 joints of pipe are above the stuck point. The pipe is hopefully unthreaded at the joint above by operating the rotary drilling rig in the opposite direction. Regrettably, unthreading can occur at any of the 299 threaded connections joining the 300 joints of pipe. It is highly desirable that unthreading occur at only one location. One approach heretofore has been to position a smaller pipe inside the drill pipe equipped with a ratchet mechanism having teeth which grip the inside of the drill pipe. This smaller pipe is rotated in the wrong direction to unthread the drill pipe. However, the smaller pipe yields somewhat due to elasticity of the metal of the small pipe string. It is therefore difficult to control the location of unthreading. The present disclosure is directed to a tool which can be lowered to any particular depth in the stuck pipe to thereby unthread the two pipe joints making a threaded connection in the drill string.
The two pipes are thus unthreaded with the assistance of the present apparatus. It operates so that unthreading torque is applied just at the single threaded connection. Relatively speaking, because either the pin or the box can be viewed as fixed, the other is rotated in the opposite direction to assure unthreading. This present apparatus spans that threaded connection and is therefore able to limit unthreading to that particular location. This can be accomplished without applying any torque whatsoever to the other portions of the drill string so that the remaining 298 threaded connections are not unthreaded. The unthreading apparatus of the present disclosure thus accomplishes unthreading at a particular location.
One of the helpful and preliminary steps is to determine the location at which the pipe is stuck. A free point indicator can be used to determine the point at which the drill string is stuck. Typically, sticking occurs in a region or at a location that can be precisely located. The present apparatus is therefore a system which can be lowered inside the stuck drill string and lowered to the region of sticking. It is lowered to that depth and then raised to the next joint thereabove. That is, it is finally located in the drill string where it straddles the pin and box threaded joint immediately above the stuck location so that unthreading can be accomplished at that joint. The present apparatus is ideally lowered on a relatively small gauge tubing string sometimes known as a spaghetti string and it is connected with the present apparatus to deliver fluid under pressure. By means of an appropriate valving apparatus fluid is applied first to one end and then to the opposite end of the tool to initiate reciprocation of an elongate shaft therein. This shaft is cooperative with upper and lower mandrels. These mandrels are threaded together at a threaded connection which has the same pitch as the pin and box connection making up the drill string. However, the upper and lower mandrels thread together with a larger number of threads so that the ultimate step of unthreading is accomplished between the pin and box in the drill string matched by unthreading in the present apparatus which, however, does not come apart because the number of threads in the threaded connection between the upper and lower mandrels is greater.
A valve is included which provides fluid for reversing the position of the shaft. As the shaft reciprocates to and fro, unthreading between the upper and lower mandrels occurs in ratcheting motion with a specified angular rotation for each stroke of the shaft. The upper and lower mandrels are gripped by wedging members, sometimes known as dogs, which hold against the drill string above and below the threaded joint so that unthreading is accomplished. After unthreading is finished, an upward pull on the drill string retrieves the unthreaded portion above the unthreaded joint and leaves the stuck lower portion. The tool of the present disclosure is released, unlocked, and retrieved from the borehole.
The present apparatus straddles a particular threaded joint to assure that unthreading occurs at that location. Unthreading is accomplished without applying any torque to the drill string from the surface. Unthreading is accomplished at the selected joint by means which applies the unthreading torque immediately adjacent to the joint, reversing the connection for unthreading to assure safety. This avoids the risk of partially unthreading many joints along the unstuck portion of the drill string.